1. Field of the Invention
The present invention relates to a process for producing a hot-rolled steel sheet having a particular composition and having a high r value in the hot-rolled state.
2. Description of the Related Arts
Deep drawing is a fundamental forming technique in sheet forming, and a deep drawability, is an extremely important factor in the formability of a sheet. As is well known, the ruling factor for a deep drawability is the r value. Attempts have been made, therefore, to enhance the r value by controlling the texture by, for example, subjecting a steel sheet to cold-rolling and annealing. The r value is calculated by the equation: EQU r=(r.sub.L +r.sub.T +2r.sub.D)/4,
in which r.sub.L, r.sub.T, and r.sub.D are the Lankford values ##EQU1## in the parallel direction, the transverse direction, and the 45-degree direction to the rolling direction, respectively.
It has been established that the r value of a hot-rolled steel sheet is low (.about. 0.9). This is because the crystal orientation in the as hot-rolled state is random and, hence, a texture advantageous for the r value cannot be obtained. Accordingly, to obtain a steel sheet having a high r value, cold-rolling and annealing are necessary. It has been believed that this cold-rolling and annealing is the only method that can be used for ensuring the r value.
Requests by the users for an enhanced formability of a hot-rolled steel sheet increase steadily, but the formability of a hot-rolled steel sheet attained by the conventional processes has reached a limit and, hence, such requests cannot be met by these conventional processes. The necessity, therefore, has arisen for the development of a hot-rolled steel sheet having novel properties.
There are a number of production methods used for producing a hot-rolled steel sheets and imparting a high formability to the sheets, in which the solute [C] and [N], which are detrimental to the aging property, are removed by the vacuum-degassing method or fixed by Ti, Nb, B, Al, and the like, thereby attaining IF (interstitial free), P and S, which are detrimental to the forming, are reduced as possible, and such IF steel with low P and S is rolled under an appropriate hot-rolled condition, thereby producing a hot-rolled steel sheet having a low YP, a high elongation, and improved, stretch flange- and bulging-properties. The r values attained by these production methods are approximately 0.90 at the highest, and these methods cannot provide a hot-rolled steel sheet having a high r value.
The processes for producing a hot-rolled steel sheet having a high r value known, for example, in Japanese Unexamined Patent Publication No. 59-226,149 and Japanese Patent Application No. 59-124,751, involve warm-rolling (+oil lubrication) during the finishing rolling stage of the IF steel. However, in these processes, the annealing is indispensable and the rolling method is technically very difficult.